Short answer - about 9-12 months on average.
What to do: Totally up to you. You can keep her in this seat until she outgrows it, and then upgrade it to a new one, or you can move her to a new one now. Just make sure you keep her rear facing. There are lots of seats on the market which will keep her rear facing and sit more upright than this one, which she might like.
Long answer (can ignore if you like) 
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Those seats are outgrown by height and weight rather than age, so you can look on the underside of it to find a weight or height limit, this will be on the orange approval sticker. They vary quite a bit in size and suitability.
Do you know your DD's height and weight now? How much space does she seem to have in the seat at the moment? Are the straps already on or close to the highest setting, or not?
If it's the older (but still legal) regulation R44 then it will just be weight based, so 13kg or 10kg.
A 10kg limit seat will last until around 9-10 months
13kg limit seats are usually outgrown physically before the weight limit is reached.
If it's the newer regulation R129, then it will have a height limit. Height limit is more restrictive usually.
75cm is usually the lowest - these seats again last until about 9 months.
80cm is a bit better, and will last until about 12 months.
Anything over 80cm, you'll usually physically outgrow the seat first.
Physically outgrowing the seat happens when the baby's head starts to poke out over the edge of the seat. Basically if you put a book or your arm over the top of the seat and it doesn't touch her head, then she's still OK in it.
If the seat has a headrest that you can move up and down to adjust the straps, then her shoulders also need to fit under the headrest. If you can't move it up any more and her shoulders are coming past the bottom of it, it's outgrown.
If you have a child on the smaller side, and/or a seat which is quite long in the shell and has decent limits on it, then it's possible to keep using this type of seat up to about 18 months, even sometimes about age 2 or even longer. But this totally depends on the seat and the child.
The other thing that tends to happen around your DD's age or a bit older is that babies this age learn to sit up more and they get a bit fed up in this kind of seat, so a lot of parents prefer to move them to the next type of seat so that they are sitting higher up and in a more upright position. It also becomes a bit cumbersome to carry them in the seat as they are heavier and their legs are hanging off the end (this is not a safety problem - it's totally fine to keep using the seat. Just convenience.) So a seat which is permanently fixed in the car can become more convenient, unless you don't have your own car. The next stage of seat is not compatible with strollers.
One thing to be aware of is that 8 month old babies definitely need to stay rear facing, whether they have moved to the next stage of seat or not. There are some seats on the market still which allow forward facing from 9kg but this is not a good option. You need to keep her rear facing until a minimum of 15 months but the safest option is to rear face longer. You can get rear facing seats which last up to around age 4 or around age 7.